1. Field of the Invention
This invention relates generally to the field of Digital Service Units (DSU) for use in conjunction with Digital Data Systems (DDS) such as the Dataphone.RTM. Digital Service provided by the American Telephone and Telegraph Company. More particularly, this invention relates to a Digital Service Unit having secondary channel noninterruptive diagnostics capability and which is compatible with both Digital Data Systems. For purposes of this document, the original Digital Data System will be referred to as DDS-I. The newly proposed Digital Data System having secondary channel capability will be referred to as either DDS-II or DDS-SC. The term DDS will be used as a generic to both types of Digital Data Systems. (Note: As of this writing, DDS-II is a proposed system which is not yet available commercially.) Also, the present DSU should be understood to include a Channel Service Unit (CSU) in the preferred embodiment and is referred to frequently in the literature as a DSU/CSU.
2. Background of the Invention
It is desirable to provide secondary channel capability to the digital data system in order to provide noninterruptive diagnostics capability to reduce down time in the system. Such a need has been recognized by the Americal Telephone and Telegraph Company which has in response proposed a new Digital Data System having inherent secondary channel capability. This proposed system is referred to as DDS-II or DDS-SC. The proposed network utilizes an eight bit frame format for data transmitted over the system between subrate DSUs (9 bits for 56 KBPS). Synchronous data is transmitted in a frame format shown in FIG. 1 in which the first six bits (seven for 56 KBPS) (D) are data bits. The seventh bit (F) is used as a framing bit and contains the repeated pattern 101100. The final bit (S/C) is shared between secondary channel use and control functions. This S/C bit is used to indicate that the D bits are data or control information. This final bit is capable of use is providing secondary channel information. When used as a control bit it provides an indication that the data bits are either control or primary channel bits. As with the standard DDS-I service, alternate mark inversion (AMI) encoding is utilized so that the data is bipolar in nature. A thorough description of the proposed DDS-SC system may be found in the references cited above.
The existing DDS system, i.e., DDS-I, imposes no frame format on the synchronous data transmitted over the system. Alternate mark inversion (AMI) is also used in the DDS-I system. In alternate mark inversion, pulses of alternating polarity are used to represent binary ones. Binary zeros are represented by a zero voltage level. In both systems, AMI is used in order to reduce the bandwidth requirements of the transmission lines as well as to assure a DC voltage level of approximately zero volts on the average across the transmission lines.
FIG. 2 shows an example AMI encoded pulse train as might typically be encountered in a DDS-I circuit. This figure shows that either a positive or negative pulse is interpreted as a binary 1 while zero volts is interpreted as a binary 0. FIG. 2 also illustrates the concept of bipolar violations in that the right-most pair of pulses are adjacent and have the same polarity. Similarly, the middle pair of pulses constitute a bipolar violation in that they also have the same polarity. This scheme is utilized to encode control signals in DDS-I networks in a manner well known in the art.
It is clear that diagnostics are desirable in a network using the Digital Data System. However, diagnostics capabilities have heretofore been available for DDS-I circuits only in the form of interruptive diagnostics where the system is incapable of normal data traffic while diagnostic activities are carried out, or by utilizing a separate statistical or time division multiplexer to provide a secondary channel. In order to provide noninterruptive diagnostics, a direct substitution of DDS-SC circuits is proposed but may not be a solution which is appropriate for all users. The cost of such service has yet to be established and it is therefore not clear whether the DDS-II service will be more economical than DDS-I or analog telepone lines for that matter. It is also not clear, in light of the divestiture of AT&T that DDS-II will be available universally throughout the continental United States as well as foreign countries. There is also no present system for utilizing DDS-II and DDS-I in the same data communication network while allowing retention of noninterruptive diagnostic capability throughout the entire network. In order to fully serve the data communication needs of the public, network transparent diagnostics is clearly desirable. It is also clearly desirable to provide a mechanism for providing DDS-I with noninterruptive diagnostics capability so that diagnostics may be used in areas which may not have DDS-SC service. The present invention addresses these needs in a single DSU thus reducing the overall cost of universal DDS compatibility while providing Secondary Channel capability.
It should also be noted that some users may be reluctant to purchase DDS-II equipment prior to the availability a DDS-II service. It is therefore desirable to provide a digital service unit which is capable of use with either DDS-I or DDS-II and which may provide immediate diagnostics capability regardless of the network. The present invention allows the DDS user to purchase diagnostic capable equipment for use on DDS-I networks and allows for easy upgrades to DDS-II in the future if available.
Other problems are also addressed with the present invention. For example, the present invention also provides a method and apparatus for correction for DC offset due to parts variation in operational amplifiers used in the Digital Service Unit as well as a technique for stabilizing the reference frequency of a voltage controlled oscillator used to recover clock information which is critical to the operation of synchronous data systems such as the digital data system. These and other problems associated with the establishment of a new DDS system are addressed by the present invention.